The pH and organic matter content of the 4-month-old soil-based nursery media
There were differences in the pH of the seven nursery media 4 months after formulation (Table 3). The values were high, with the highest values of 8.60 recorded for topsoil + droppings + sawdust, the reference medium of this study. This was followed by topsoil + droppings + huskdust and topsoil + dung + sawdust. The lowest pH values of 6.83 were recorded for both topsoil + dung + huskdust and the control. These two media thus showed pH values closest to the range of 5.50–6.50 usually considered as optimum for seedlings in nursery media (Goh and Haynes 1977; Miller and Jones 1995). The pH is an important index of chemical quality and horticultural value of nursery media. For instance, Salifu et al. (2006) reported that pH alone explained 54% of variation in height of Quercus rubra seedlings in three variously formulated soilless media. The favourable pH of the topsoil + dung + huskdust may thus be considered its advantage over the other media formulations.
The presence of rice-husk dust in topsoil + dung + huskdust would partially explain the near-neutral pH found in this nursery medium (Baiyeri and Mbah 2006). From the present results, it appears that adding cattle dung and rice-husk dust to topsoil to serve as manure and aerator, respectively, in nursery media is ineffectual in increasing the soil pH. Considering that the pH shown by the control is a ‘near-neutral’ value usually preferred by seedlings of most plants (Goh and Haynes 1977), it could be that the combination of cattle dung and rice-husk dust has the potential to buffer the pH of the nursery medium against increases beyond this optimal value. Notably, the replacement of sawdust known to be the conventional aerator with rice-husk dust caused a ‘desirable’ reduction in the pH of all media except the ones with pig slurry as organic manure where the opposite prevailed.
All the formulated nursery media showed increases in organic matter content compared with the control; topsoil + slurry + huskdust was the only one where the increase over the control was marginal (Table 3). The highest values were recorded for topsoil + slurry + sawdust and topsoil + dung + huskdust. Therefore, the similarity between topsoil + slurry + huskdust and the control is rather surprising. It could be, however, that pig slurry is more compatible with sawdust and cattle dung with rice-husk dust regarding providing nursery media with aeration that favours certain biochemical processes that enhance organic matter status in manure-amended soils. In what seems like an inverse of the trend for pH, the replacement of sawdust with rice-husk dust caused organic matter to increase in all those soil-based media amended with poultry droppings (marginally though) and cattle dung; the opposite prevailed in those amended with pig slurry.
Total nitrogen and available phosphorus in the 4-month-old soil-based nursery media
The results for percent total nitrogen somewhat mirrored those for organic matter content (Table 3). Values were highest in topsoil + slurry + sawdust and topsoil + dung + huskdust and lowest in the control, while the increment in topsoil + slurry + huskdust over the control was marginal. Nursery media content of total nitrogen has, however, been shown not to be a good measure of their fertility status as evaluated by seedling performance; the topsoil + slurry + sawdust and/or topsoil + dung + huskdust would be deemed superior to topsoil + slurry + huskdust showing lower values only if they also show higher contents of other macronutrients including available phosphorus (Asiah et al. 2004). Also, the five nursery media formulations showing similar contents of total nitrogen here may differ when evaluated by seedling performance because both net nitrogen mineralization and net nitrogen immobilization are possible (Clark and Cavigelli 2005). The replacement of sawdust with rice-husk dust caused marginal increases in percent total nitrogen of all media except those amended with pig slurry where the replacement caused a significant reduction instead.
Also, there were differences in available phosphorus content of the various nursery media (Table 3). The topsoil + droppings + huskdust recorded the highest values while the control recorded the lowest values. Low levels of other plant nutrients in nursery media from germination through the early weeks of seedling growth could be tolerated because nutrient requirements during this stage are minimal, but not for phosphorus (Miller and Jones 1995). Available phosphorus being highest in topsoil + droppings + huskdust is thus an edge this formulation has over the rest of the nursery media. The trend due to the replacement of sawdust with rice-husk dust (higher values in the nursery media amended with poultry droppings and cattle dung and corresponding lower values in those amended with pig slurry) subsisted; however, the replacement induced marginal reductions this time around.
Cation exchange properties of the 4-month-old soil-based nursery media
The data for the exchangeable bases and acidity of the various soil-based nursery media 4 months after formulation are shown (Table 4). The content of K+ was higher in all the formulated nursery media relative to the control. There were no differences in either of the droppings-amended two media and the slurry-amended two media due to replacement of sawdust with rice-husk dust, but the dung-amended counterparts differed, with topsoil + dung + sawdust showing higher K+ than topsoil + dung + huskdust. Perhaps, aerator replacement had no effect on aeration porosity of the droppings-amended and the slurry-amended media (data not yet processed, hence not shown); otherwise, this observation implies that the capacity of the concerned soil-based media to supply potassium is not influenced by their degree of aeration.
The Ca2+, apart from its being higher in the reference medium compared to topsoil + slurry + huskdust, indicated similar values among the seven soil-based media. Hence, the replacements done in the reference medium (i.e, replacing poultry droppings with pig slurry or cattle dung as organic manure and sawdust with rice-husk dust as aerator), when considered individually, produced no effect on the Ca2+ content of the soil-based media; differences were due to simultaneous replacement of poultry droppings with pig slurry and sawdust with rice-husk dust.
Treatment also affected Mg2+ of the nursery media with highest and lowest values in topsoil + droppings + huskdust and the control, respectively (Table 4). Contrary to the observation for pH, replacement of sawdust with rice-husk dust caused increases in Mg2+ content of droppings-amended and dung-amended soil-based media but decreases in that of slurry-amended ones. Because of the positive role of Mg2+ in seedling performance in the nursery (Baiyeri and Aba 2013), it can be considered an important fertility index of nursery media. For the droppings-amended media, therefore, the significant improvement in Mg2+ over the reference medium by topsoil + droppings + huskdust may have some agronomic relevance. Notably, topsoil + slurry + huskdust and topsoil + dung + sawdust were so low in Mg2+ that they had similar values as the control, thus pointing to the aforementioned compatibility of pig slurry with sawdust and not rice-husk dust and of cattle dung with rice-husk dust and not sawdust in the formulation of soil-based nursery media.
The Na+ content of the nursery media was also affected by treatment. All the formulated media recorded higher values compared with the control. There were also differences among them, with the highest values in topsoil + dung + sawdust, plausibly due to the use of soapy materials for washing in abattoirs. Both the replacement of manure and aerator contributed to these differences. For the aerator replacement, increases in Na+ due to the change from sawdust to rice-husk dust were recorded for the droppings- and slurry-amended media; decreases occurred only for the dung-amended media. The decreases were such significant that, among the variously blended media, the highest values in topsoil + dung + sawdust dropped to lowest in topsoil + dung + huskdust. Considering the widely known dispersive tendency of sodium in soils and the fact that it is not an essential but functional plant nutrient (Maathuis 2014), the results just stated favour the adoption of topsoil + dung + huskdust in nursery practice. The topsoil + slurry + sawdust was similar to topsoil + dung + huskdust in Na+ content. In the context of doubtful utility of excessive sodium in soils, this observation lends credence to the aforementioned compatibility of pig slurry with sawdust and of cattle dung with rice-husk dust in the formulation of soil-based nursery media.
The exchangeable acidity of the soil-based media was dominated by H+, with Al3+ occurring in traces. Higher values of the H+ were recorded in the reference medium relative to the rest of the nursery media. However, the H+ tended to decrease in the order droppings-amended media, slurry-amended media, dung-amended media, and the control.
The aggregated effect of the differences in the exchangeable bases and H+ in the e-CEC of the soil-based media shows that all the formulated media but the topsoil + slurry + huskdust were superior to the control. However, all treatments showed e-CEC of above 10 cmol kg−1 regarded as the minimum for nursery media (Miller and Jones 1995). Reference medium and topsoil + droppings + huskdust had similar e-CEC, values of which were higher than the rest of the treatments. By contrast, the replacement of sawdust with rice-husk dust as aerator caused an increase in the e-CEC of both the slurry-amended and the dung-amended media. That the change of aerator from sawdust to rice-husk dust affected not the e-CEC of droppings-amended media but that of slurry- and dung-amended media suggests that the complementary role of rice-husk dust in enhancing the fertility of nursery media could be masked in the presence of such nutrient-rich sources of manure as poultry droppings.
But for the higher values in the reference medium than topsoil + droppings + huskdust, the results for the apparent CEC of the soil-based media were somewhat congruent to those for the e-CEC. Generally, the media formulations of this study were of low cation exchange capacities. This is despite the increases in their organic matter content due to the organic manures involved in the formulation. It could be that the formulations were yet to attain full decomposition and mineralization at the time of sampling which took place 4 months after formulations. Another plausible reason is that texture seems to have stronger influence than organic matter on CEC of the soil used for the formulations (Obalum et al. 2013).
The change of the aerator from sawdust to rice-husk dust resulted in the percent base saturation being higher in droppings- and dung-amended media compared to slurry-amended media which together with the control showed the lowest values. Also, neither of the droppings-amended and the dung-amended media differed in percent base saturation due to the change of the aerator from sawdust to rice-husk dust, unlike the slurry-amended media in which the replacement lowered the percent base saturation, hence the lowest values recorded in topsoil + slurry + huskdust.